Furnace for burning bulky fuels, especially refuse, and improved method for operating the aforesaid furnace



Aug. 8, 1967 R. TANNER 3,334,599

FURNACE FOR BURNING BULKY FUELS, ESPECIALLY REFUSE, AND IMPROVED METHOD FOR OPERATING THE AFURESAID FURNACE Filed Nov. 25, 1964 ,4 TTOIQNEYS United States Patent 3,334,599 FURNACE FOR BURNING BULKY FUELS, ESPE- CIALLY REFUSE, AND IMPROVED METHOD FOR OPERATHNG THE AFORESAID FURNACE Richard Tanner, Zurich, Switzerland, assignor to Von Roll AG, Gerlafingen, Switzerland Filed Nov. 25, 1964, Ser. No. 413,755 Claims priority, application Switzerland, Nov. 27, 1963, 14,548/63 6 Claims. (Cl. 110-8) The present invention relates to an improved furnace for burning on a grate bulky fuels of varying chemical and physical properties, for example domestic refuse, as well as to an improved method for operating the aforesaid furnace for carrying out the burning of such bulky fuels.

Heretofore known furnaces for the burning of bulky fuels of varying quality, for example domestic refuse, have operated mainly on the on the principle of ignition from above. As is known, these furnaces have forced-feed grates, travelling grates, roller grates, or the like. The reason for the widespread sue of such grate furnaces is that fuels of the type specified hereinbefore generally have a considerably tangled structure. Consequently, the fuel newly introduced into the furnace can scarcely ever be moved through the combustion chamber other than in the form of a relatively dense, coherent layer, all parts of which move at the same speed. These conditions prevail until the progressive burning has loosened the cohesion of the fuel bed to such an extent that the structure of the fuel can be regarded as lumpy.

The system of igniting from above, which has already been mentioned, operates in the following manner:

After the usual, at least local pre-drying of the fuel by heat radiation from the brickwork and hot gases in the combustion chamber, the uppermost layer of the fuel bed lying on the combustion grate is heated to ignition point so that combustion is initiated, the necessary oxygen being supplied by undergrate air blast. Due to the conduction and radiation of heat the burning zone spreads downwardly within the fuel bed, until the bed has been ignited over its entire depth.

However, a disadvantage of this known system is that the speed at which the downwardly spreading ignition is propagated within the bed of fuel is relatively slow, so that the initial portion of the combustion grate has only a relatively low combustion efficiency.

The already-mentioned pre-drying of the fuel is effected generally on a separate predrying grate and by heat radiation from the flames in the combustion chamber and/or the red-hot, refractory walls and roof of the chamber. This drying process is assisted or intensified, if appropriate, by hot gases, for example pre-heated air and/or re-cycled hot flue gases, blown from below through the pre-drying grate.

Nevertheless, the pre-drying effect is often inadequate and, therefore, owing to the incomplete pre drying of the fuel, the already relatively slow speed at which the ignited zone is propagated downwardly in the bed of fuel is further decreased.

The ignition temperatures for certain fuels in question (such as cellulose, plastics material, rubber, and scrap leather) are in the region of 300 C. to 500 C. However, these temperatures are reached only after complete, at least local, drying since when heat acts on such fuel which is still moist the temperature does not rise much above 100 C. owing to the need to evaporate the moist contents. As a result, of course, the downward propagation of the ignition zone is hindered.

Thus, a primary object of the present invention is to provide an improved furnace for the burning of fuels of 3,334,599 Patented Aug. 8, 1967 the type specified initially and, more specifically, a furnace with a mechanical grate wherein the aforesaid disadvantages of the known burning techniques are obviated.

Another important object of the present invention relates to an improved method of burning on a grate bulky fuels of varying chemical and physical properties, such as domestic refuse for example.

According to one aspect of the present invention, there is provided an improved method of burning bulky fuels in a furnace which are of varying chemical and physical properties, for example domestic refuse, comprising the steps of: causing the fuel to travel as a bed on a combustion grate, igniting said bed from above, supplying to a location beneath an initial portion of said grate hot gas at a temperature at least sufficient to ignite a substantial proportion of said fuel on said initial portion, and passing said hot gas upwardly through said initial portion and through the fuel on said initial portion, to thereby ignite from below at least a substantial proportion of the fuel on said initial portion.

According to another aspect of the present invention, there is provided a furnace for use in carrying out the inventive method, comprising a combustion chamber, a combustion grate at the bottom of said chamber, means for causing said fuel to travel as a bed on said combustion grate, and supply means which serves to supply to a location beneath an initial portion of said combustion grate hot gas at a temperature at least suflicient to ignite a substantial proportion of the fuel on said initial portion, said initial portion being formed of a material capable of withstanding said temperature.

Most preferably, secondary air may be introduced above and laterally of the fuel bed on the initial portion of the combustion grate for the purpose of burning the volatile constituents which are expelled from the fuel on the initial portion of the combustion grate.

When using a grate furnace having a pre-drying grate, some of the hot gas in the combustion chamber may be drawn downwardly by suction through the pre-drying grate and through the still moist fuel thereon in order to pre-dry the fuel.

The operation of grate furnaces with pre-heated undergrate blasts has already been known to the art for some time. By this means, the pre-heated undergrate blast not only furnishes the oxygen required for combustion but also a certain quantity of sensible heat, whereby a higher temperature is achieved in the combustion chamber and thus the combustion is assisted. Since the heat necessary for pre-heating the undergraduate blast is conventionally taken from the furnace at a location downstream of the combustion grate with reference to the flue gas stream, whether by air preheaters arranged in the flue gas stream or by partial re-cycling of the flue gas, the overall thermal efiiciency of the furnace is increased by the use of preheated undergrate blast.

In contrast, however, the burning method of the furnace according to the present invention, is founded on the basic consideration that the ignition and combustion efiiciency of the initial portion of the combustion grate could be increased by initiating ignition of the fuel bed from below simultaneously with and in addition to the ignition of the fuel bed from above by heat radiation from the combustion chamber.

This consideration led to the further realisation that, by using a gaseous heating medium of very high temperature, for example intensely heated air and/or re-cycled hot flue gas, and by conducting the medium from below through the initial portion of the combustion grate into the fuel bed, it is possible to heat the lowermost fuel layer, i.e. that which is situated directly on the grate, to or even above its ignition temperature.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made by way of example to the accompanying drawing which shows a diagrammatic representation of a furnace for use in the described method of burning bulky fuels.

In the single figure the furnace is in this case an incinerator, and the fuel is assumed to be domestic refuse for example. The fuel is introduced into the incinerator through a charging shaft 1 and then arrives at a regulata-ble feed device 2 which introduces the fuel into the actual combustion chamber 3 of the incinerator, the walls of which are covered in a suitable manner with refractory material.

In the combustion chamber 3 there are a pre-drying grate 4 and a combustion grate 5, the latter being imagined as subdivided into two grate portions, namely an initial portion 5a and a final portion 5b. The two grates 4 and 5, which can be of any mechanical continuousfeed type, are disposed one before" the other with an inclination in the same sense. The inclination and construction of the grates is such that conveyance of the fuel as a bed during combustion is obtained. Between the end of the pre-drying grate 4 and the beginning of the combustion grate 5 there can be provided a vertical step 6 which ensures that the fuel-bed is subjected to a free-fall on passing to the combustion grate 5, and thereby is loosened-up mechanically. On the final portion 5b of the grate 5, complete burn-out of the fuel is achieved, the non-combustible ash being discharged through a shaft 7 disposed after the aforesaid final portion 51;.

A very hot blast, consisting of air either directly preheated or mixed with re-cycled hot flue-gas, is blown through the initial grate portion 5a and the fuel bed 8b lying on it, the temperature of the blast being equal to or, if appropriate, even higher than the ignition point of a substantial proportion of the fuel on this initial grate portion 5a. This hot gas stream delivered by the supply means 18, and which in the drawing is represented schematically by arrows 9, 9 and 9", and which is distinct from the usual underblast such as is used for the final portion 5b and is indicated in the drawing by arrows 10, 10' and 10", penetrates upwardly through the fuel bed 8b from below, whereby the fuel is ignited not only from above by heat radiation from the combustion chamber 3, but at the same time from below, with the result that extremely intensive combustion .is achieved, with rapid gasification of the volatile constituents of the fuel.

The volatile constituents, issuing intensively from the fuel in the form of combustible gases in the region of the portion 5a, are ignited above the fuel bed 812. For the combustion of these constituents, a plurality of nozzles 11 are arranged in the two side walls 12 (of which only one is seen) of the furnace, above and laterally of the grate portion 5a, to blow secondary air into the combustion chamber 3, thereby supplying the necessary oxygen for the final combustion of the combustible gases, and also exerting an intensive whirling action on the combustible gases, so that a continuous flame forms above the aforesaid grate portion 5a in the combustion chamber 3. The supporting and driving elements of the grate portion 5a which are acted upon by the hot gas stream 9, 9' and 9" are of adequately adapted design and construction, to withstand the high temperature of the hot gas stream which may be from 300 C. to 600 C., depending upon the nature of the fuel.

The pre-drying of the fuel is effected, on the pre-drying grate 4, on the one hand by heat radiated from the flames and from the red-hot, refractory walls and roof of the combustion chamber 3, and, on the other hand, by the fact that hot combustion gases from the combustion chamber 3 are sucked downwards through the pre-drying grate 4 and through the still moist fuel-bed 8a lying thereon, as is indicated schematically by arrows 13, 13 and 13" in the drawing, into a negative pressure chamber 16 situated therebelow. For this purpose, in this negaf tive pressure chamber 16 beneath the pre-drying grate 4, which is sealed laterally in a suitable manner relatively to the two side walls 12 of the furnace (not shown), a negative pressure is being maintained by connection to the suction side of at least one suction and pressure fan S arranged in a return conduit 17.

The vapors produced by evaporation during the predrying phase are removed by suction below the pre-drying grate 4 along with the hot combustion gases from the combustion chamber 3 and are returned into the furnace at a location downstream of the combustion chamber 3 where they cannot disturb the actual burning operation and the burn-out of the combustible gases, but where, however, the temperature is still sufficiently high that these vapors can be effectively deodorized. Accordingly, the extracted stream including the vapors, and indicated at 14, is conducted through the return conduit 17 into the furnace at a point 15 which is situated downstream of the combustion chamber 3 and just before the discharge flue 19. As can be seen, the point 15 is remote from the grates 4 and 5 and from the combustion process taking place on them.

If appropriate, a single fan S can be provided for drawing the combustion gases 13, 13 and 13" through the predrying grate 4 and also for returning the extracted stream to the chamber 3 at the point 15.

Owing to the fact that the temperature of the hot gas stream 9, 9' and 9 is extremely high, the primary effect of this gas stream is directly to ignite the lowermost layer of the fuel-bed, namely these particles which are first impinged upon by the hot underblast. Thus, the fuel bed 8b is not only ignited from above by heat radiated from the combustion chamber 3, but at the same time from below by the formation of ignition nuceli at the underside of the fuel bed 8b. In this way, it is achieved that the fuel begins to burn not only at its uppermost surface but also at its lowermost surface. Consequently, the fuel bed 8b is ignited throuhout its entire bed thickness after only a short travel along the combustion grate 5.

A second result of blowing-in gas heated to a high temperature is that by the combined effects of the firing from below and of the sensible heat still contained in the hot underblast the gasification of the fuel bed 8b is being considerably increased. The intensive expelling of the volatile constituents from the fuel considerably reduces the fuel volume and also effectively loosens-up the structure of the fuel bed, giving more advantageous burning conditions for the burning-out of the fuel on the grate portion 512.

It will be apparent that these two effects described hereinbefore go hand-in-hand and are extremely advantageous to the entire combustion process. These phenomena considerably increase the maximum grate output rate by increasing the permissible grate loading at the beginning of the combustion grate. Moreover, they enable the required grate surface area to be considerably reduced.

As a third effect of the hot underblast, the introduction of additional heat into the combustion chamber should also be mentioned, in the manner of the hitherto conventional fire operating method. Although such an effect is per se not essential for the present combustion method, there should be stressed the advantageous possibility that this known feature of supplying heat in this way can readily be used with the present method of operating the furnace. The present measure of blowing-in gas at an extremely high temperature beneath the initial part of the combustion grate in order to intensify combustion may also be used in cases where no provision is made for predrying by means of hot combustion gases taken from the combustion chamber and drawn by suction downwardly through the fuel bed.

While there is shown and described a present preferred embodiment of the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims.

What is claimed is:

1. A method of burning in a furnace domestic and industrial refuse of bulky condition and varying physical and chemical properties, said method including the steps of:

placing the total refuse to be burned into the actual combustion chamber of the furnace;

forming said total refuse into only a single layer of substantially uniform thickness;

pre-drying said single refuse layer on a pre-drying grate within said combustion chamber;

transferring said single refuse layer onto a combustion grate, said combustion grate including initial and final portions;

supplying hot gases to a location beneath said initial portion of said combustion grate, said hot gases having a temperature at least equal to the ignition point of a substantial portion of said single refuse layer; passing said hot gases upwardly through said initial portion of said combustion grate and completely through said single refuse layer to thereby ignite from below at least a substantial part of said single refuse layer on said initial combustion grate portion; introducing secondary air laterally into said combustion chamber at a location above said initial combustion grate portion so as to burn at said location volatile constituents expelled from said single refuse layer upon said initial combustion grate portion; and completely burning the single refuse layer disposed upon said final portion of said combustion grate.

2. A method as defined in claim 1, wherein the step of pre-drying said single refuse layer further includes:

supplying hot combustion gases from said combustion chamber of said furnace to a location above said predrying grate;

passing said hot combustion gases downwardly through said pre-drying grate and said single refuse layer disposed thereon;

removing by suction below said pre-drying grate vapors produced by the passing of said hot combustion gases through said single refuse layer; and

returning said vapors into said combuston chamber at a location remote from said pre-drying grate.

3. A method as defined in claim 1, wherein said hot gas temperature is approximately between 300 C. and 600 C.

4. A furnace for burning domestic and industrial refuse of bulky condition and varying chemical and physical properties, comprising:

combustion chamber means;

combustion grate means within said combustion chamber means for receiving refuse thereon;

means for causing all of said refuse to travel only as a single layer on said combustion grate means, said combustion grate means including an initial grate portion and a final grate portion;

supply means for supplying to a location beneath said initial grate portion of said combustion grate means hot gas at a temperature at least equal to the ignition point of a substantial portion of the single refuse layer disposed upon said initial grate portion;

said supply means further producing a flow of said hot gas upwardly through said initial grate portion of said combustion grate means and completely through said single refuse layer disposed thereon;

nozzle means for introducing secondary air laterally into said combustion chamber means above said initial grate portion of said combustion grate means; and

said combustion chamber means perfecting complete burning of the single refuse layer disposed upon said final portion of said combustion grate means.

5. A furnace according to claim 4, further comprising:

a pre-drying grate disposed ahead of said combustion grate means;

said causing means further insuring that all of said refuse travels only as a single layer on said pre-drying grate;

a negative pressure chamber disposed beneath said predrying grate and bonded at the top thereby; and suction means including at least a single suction and pressure fan, said suction means having a suction side connected to said negative pressure chamber and a pressure side connected to said furnace at a location remote from said pre-drying grate and combustion grate means and down-stream of said combustion chamber means with respect to the path of the flue gas stream in said furnace. 6. A furnace as defined in claim 4 wherein said hot gases supplied by said supply means are at a temperature approximately between 300 C. and 600 C.

References Cited 'UNITED STATES PATENTS 1,531,766 3/1925 Trimborn et al -8 2,032,412 3/ 1936 Greenwald 11015 2,057,681 10/ 1936 Harrington 110-15 2,752,869 7/ 1956 Keenan.

2,811,937 11/1957 Bouchard 110-49 X JAMES W. WESTHAVER, Primary Examiner. 

1. A METHOD OF BURNING IN A FURNACE DOMESTIC AND INDUSTRIAL REFUSE OF BULKY CONDITION AND VARYING PHYSICAL AND CHEMICAL PROPERTIES, SAID METHOD INCLUDING THE STEPS OF: PLACING THE TOTAL REFUSE TO BE BURNED INTO THE ACTUAL COMBUSTION CHAMBER OF THE FURNACE; FORMING SAID TOTAL REFUSE INTO ONLY A SINGLE LAYER OF SUBSTANTIALLY UNIFORM THICKNESS; PRE-DRYING SAID SINGLE REFUSE LAYER ON A PRE-DRYING GRATE WITHIN SAID COMBUSTION CHAMBER; TRANSFERRING SAID SINGLE REFUSE LAYER ONTO A COMBUSTION GRATE, SAID COMBUSTION GRATE INCLUDING INITIAL AND FINAL PORTIONS; SUPPLYING HOT GASES TO A LOCATION BENEATH SAID INITIAL PORTION OF SAID COMBUSTION GRATE, SAID HOT GASES HAVING A TEMPERATURE AT LEAST EQUAL TO THE IGNITION POINT OF A SUBSTANTIAL PORTION OF SAID SINGLE REFUSE LAYER; PASSING SAID HOT GASES UPWARDLY THROUGH SAID INITIAL PORTION OF SAID COMBUSTION GRATE AND COMPLETELY THROUGH SAID SINGLE REFUSE LAYER TO THEREBY IGNITE FROM BELOW AT LEAST A SUBSTANTIAL PART OF SAID SINGLE REFUSE LAYER ON SAID INITIAL COMBUSTION GRATE PORTION; INTRODUCING SECONDARY AIR LATERALLY INTO SAID COMBUSTION CHAMBER AT A LOCATION ABOVE SAID INTIAL COMBUSBUSTION GRATE PORTION SO AS TO BURN AT SAID LOCATION VOLATILE CONSTITUENTS EXPELLED FROM GRATE PORTION; AND COMPLETELY BURNING THE SINGLE REFUSE LAYER DISPOSED UPON SAID FINAL PORTION OF SAID COMBUSTION GRATE. 